1. Field of the Invention
This invention introduces a new concept to directly map the subsurface formations in geophysical or subsurface exploration. This technology can be easily implemented as part of a regular exploration program. Surface seismic vibrators are generally being used as a seismic source for land exploration throughout the world. For most normal seismic operations, a swept frequency signal is transmitted by the surface vibratory units and recorded by multiple detectors located either in some form of 2D or 3D configuration; it is universally practiced and known in the art. For the purpose of introducing the concept of this invention, there is no necessity to describe the conventional swept signal in detail. Also known in the art, the recorded data of the conventional method is cross-correlated with the swept frequency signal that was used as a seismic source to generate the compressional wave transmitted into the earth.
In addition to a swept frequency signal, these surface seismic vibrators can generate a monofrequency signal of any chosen frequency within the conventional seismic bandwidth. In this invention, the monofrequency signal is transmitted on its own, and is added as a separate step at every nth station, to the recording that is used for the known and conventional methods of today.
By transmitting the compressional wave in the form of the monofrequency signal from the source, this unique approach is used to directly record the signature properties of the subsurface reservoir formations whenever the reservoir is present.
Reservoirs that are porous, permeable, and fluid-saturated, display different signal transmission characteristics compared to non-reservoir formations. Since reservoirs are fluid-saturated and provide permeable connections through interconnected pores, then a part of the compressional energy is transferred. The velocity of the compressional wave depends on the rock matrix of the reservoir and also to a certain extent the pore fluids that saturate the formation. The Drag Wave travels at a slower velocity, which is slower than the velocity of the compressional wave in the pore fluid itself.
These two waves, the compressional wave and Drag Wave, both propagate through the reservoir formation simultaneously. Because of the difference in their propagating velocity, a Doppler shift in the primary frequency of the monofrequency takes place. A lower frequency is generated within the reservoir, which may be three to four times lower than the primary frequency of the originally transmitted monofrequency. This lower Drag Wave frequency depends on the tortuosity and permeability of the reservoir rock; the properties of the pore fluids in the rock; and becomes a direct signature of the reservoir formation. The value of the lower Drag Wave frequency generated and its amplitude will depend on the reservoir properties and is going to change according to the permeability of the reservoir formation, the viscosity of the pore fluids, the tortuosity of the reservoir rock, the porosity of the reservoir formation, and other reservoir characteristics (maybe the clay content).
While recording the normal conventional Vibroseis data after certain prescribed distances, we can transmit the monofrequency signal to evaluate the presence or the absence of reservoir rocks underneath that source location. The presence of this lower Drag Wave frequency, is an indicator of the presence of subsurface reservoir rocks. The absence of the lower Drag Wave frequency is also a strong indicator that there are no commercially viable reservoir rocks underneath those locations. In this manner we can identify the potential areas which will be of interest for hydrocarbon exploration, and discard the areas that do not show any potential prospect of finding any reservoir fluids in the subsurface. This simple exploration technology will eliminate drilling unnecessary wells since the absence of the lower Drag Wave frequency also indicates the absence of any fluid-saturated reservoir rocks. There is no limitation on the quantity and variety of monofrequencies that can be transmitted at every nth station.
Another important contribution of this technology is that by recording the lower Drag Wave frequency signal generated by the monofrequency transmitted from the surface, we can calculate a transfer function, or conversion factor. Once we have this ratio between the monofrequency signal and the lower Drag Wave frequency, we can convert the swept frequency signal being used for conventional seismic recording and use it for cross-correlation of the data obtained by the conventional Vibroseis sweep. For instance if the conventional swept frequency signal being used for conventional recording is 15 Hz to 60 Hz, and the transfer function is of 3, then we can generate a sweep of 5 Hz to 20 Hz which will represent the Drag Wave frequency when the normal conventional Vibroseis sweep is transmitted through the reservoir rock. The newly created sweep will be cross-correlated with the conventional data set to obtain an image that will show the extent and location of the reservoir.
This reservoir-generated lower Drag Wave frequency is limited to the reservoir and comes to an abrupt end at the lower interface of the reservoir formation since there is no permeability beyond that point. Due to this abrupt termination at the lower reservoir interface, a strong reflected signal is generated. This strong reflected signal is of the lower Drag Wave frequency related by the transfer function determined while using the monofrequency signal.
Any descriptive teens of equipment used, such as source, vibrator, Vibroseis, and detector are generic names for the devices that are capable of transmitting and measuring a pressure wave or another quantity, and they are known in the art. Thus, a detailed description of these devices is not provided herein.
It should be noted and understood that there can be improvements and modifications made of the present invention described in detail above without departing from the spirit or scope of the invention.
2. Description of Related Art
The seismic subsurface imaging methods of the past few decades use seismic attribute analysis and signal amplitude extraction in the subsurface structural features to map the reservoir characteristics. In spite of recent advances in seismic acquisition and data processing, the results are quite often non-unique and ambiguous. The results fail to identify the extent and location of the fluid-saturated subsurface reservoir formations. The industry needs new technologies to be developed which directly relate to the reservoir properties of interest, rather than the subsurface structure.
The most important reservoir properties that differentiate the reservoir rocks from non-reservoir rocks are porosity, permeability and the identification of the pore fluids. In large exploration programs when the seismic data volume can become very large and overwhelming, the industry needs a simple yet sophisticated method which will identify more promising leads and that will direct the explorationist to focus on the areas which are more likely to be commercially viable. This patent is designed to address that shortcoming by using simple acquisition and data processing methods to locate the subsurface reservoir formations that will provide more commercial and beneficial returns.
Those skilled in the art would understand that current conventional seismic recording involves the use of a swept signal that is generated from a vibratory source. The data recorded using the conventional swept signal is cross-correlated and this recording procedure is known in the art. There is no existing conventional seismic method of utilizing the Drag Wave and directly mapping the location and extent of the subsurface reservoirs. This Drag Wave is a unique signal that is dependent upon the attributes of permeability, porosity, fluid content and tortuosity of the reservoir rock.
In the current conventional seismic exploration recording, the Drag Wave is ignored because the cross-correlation with the conventional Vibroseis sweep acts like a powerful filter and discards all the other signals that are generated. The Drag Wave is ignored because it is outside of the spectrum of the conventional swept frequency signal generated by the seismic vibratory source.
A number of the scientists who have studied nonlinear acoustic wave propagation have performed studies and determined that the possibility exists to use these concepts to further understand the Earth's subsurface, in our case for the purposes of locating the hydrocarbon reservoirs.
Biot (1956) proposed a comprehensive theory that explained many important features of the seismic wave propagation in fluid-saturated porous media. One of the important contributions of his theory is the prediction of a Slow Compressional Wave with a speed lower than that of the rock matrix or the pore fluid. The Slow-Wave involves a coupled motion between the fluid and the solid frame. The Slow-Wave's velocity and attenuation depend on the morphology of the pore space and the pore interconnections, which also determine the reservoir properties such as permeability, porosity and the presence of pore fluids. The detection of the presence of the Slow-Wave or Drag Wave in a reservoir formation will be an indicator of the reservoir rocks.
Donskoy (1997) wrote that “Nonlinear dynamic behavior of porous media, natural or artificial, has attracted increasing attention recently. It has been observed experimentally that porous media, such as soil, rocks, sediments, etc., exhibit a strong elastic nonlinearity, in some cases two to three orders of magnitude greater as compared with nonporous media.” He also stated that the nonlinear parameters of the porous media depend on porosity.
Johnson has also suggested that the measurement of the dynamic elastic nonlinearity of the reservoir rocks is a sensitive tool because the porosity induces an orders of magnitude change for the nonlinear coefficients and a few percent change for linear parameters of velocity, attenuation etc. The “nonlinear signal” is important for investigating the nonlinear properties of rocks.
Johnson writes further to state that the ramifications of nonlinear response in rock may ultimately affect many areas of research in geosciences including seismology, where the spectral distortion of seismic waves during propagation must be considered. And, that the primary mechanisms that produce nonlinear response in rock are due to the low-aspect ratio compliant features (cracks, grain-to-grain contacts, etc.)
Klimentos (1988) stated that “Permeability is strongly dependent on pore size but is also a function of the tortuosity of the pores in a rock. Consequently, the properties of the slow wave may be of considerable practical importance in estimating rock permeability in situ.”
Meegan (1994) stated that Earth materials are an important example of this type of disordered media because of their practical importance in geophysics and seismology.
Scheidegger (1960) published a book on the physics of flow in porous media, and gives us a better understanding of the significance of the physical aspects of porous, permeable and fluid saturated reservoir rock.
Additionally, the existing U.S. Patents assigned to Nonlinear Seismic Imaging, Inc. and invented by Khan since 2001 when the company was formed, including U.S. Patent Application Publications Nos. 2002/0188407 and 2003/0004649, reflect the understanding of the concepts and its own empirical observations while performing the field evaluations. As a basis for the development of this technology, there were three main attributes that were identified that could be used as a basis of a new effort that have either been ignored or not fully understood by the geophysicists involved in the seismic exploration effort:
1. When a seismic compressional wave propagates through a reservoir formation it generates harmonics of all the primary frequencies that are present in the seismic signal.
2. When there are more than one seismic signals propagating through the reservoir formation simultaneously, the sum and difference frequencies of the two primary waves are created, and that is a unique property of the reservoir formation.
3. During the propagation of the compressional wave through the reservoir formation which is permeable and fluid-saturated, another seismic wave is created which is identified as the Slow Wave or Drag Wave. The Slow Wave travels at a lower velocity than the velocity of the compressional wave in the mineral frame of the rock, or the velocity of the compressional wave in the fluid that saturates that reservoir formation. This phenomenon creates a very low frequency wave that will only be present in the reservoir formation and not in any other subsurface rock.
Nonlinear Seismic Imaging, Inc.'s technology uses these three main characteristics to directly map the presence of hydrocarbons in the subsurface formations. To successfully achieve the desired results, data acquisition has to be specifically designed so that these seismic attributes are created and preserved for further analysis and interpretation after the seismic data have been processed. Without the proper data acquisition, these attributes cannot be usefully extracted during the data processing, and the operator will not get the desired image to identify the presence of hydrocarbons. Nonlinear Seismic Imaging, Inc. previously patented a range of methods for specific and distinctly separate objectives, which can be applied to hydrocarbon exploration and production. These methods include mapping subsurface fractures as described in U.S. Patent Application Publication No. 2003/0004649, mapping the porosity profile as described in U.S. Patent Application Publication No. 2002/0188407, mapping the changes in pore fluids in production stages, and understanding permeable flow units away from the wellbore. The current invention will add to the existing toolbox of solutions for the operators, and will provide a new and unique method to illuminate the subsurface reservoir when incorporating this technique within the conventional recording for the area of interest.